port project to sdl-2.0

The difference between SDL 2.0 and SDL 1.2 resides in the use of
textures instead of surfaces, which implies changes in the fuunctions
responsible of rendering and the use of SDL_Texture data structure.
Also, the are changes in the way SDL is initialized.
fix bug bonus in dual ronda bonus calculation

bonus should be added not affected.
Merge branch 'sdl2'